The presence of “gels” is a common problem in polyolefins. The term “gels” refers to highly localized imperfections in the polyolefin, especially film made from polyolefin, that are visually distinct from the surrounding film, mostly due to the presence of either high concentrations of unblended polymer, unreacted catalyst and activator, or both, but can include other types of visually distinct imperfections as well. The presence of gels lowers the value of these films, and in some cases makes the films unmarketable. There have been many approaches in the past to solving this problem. One approach is to remove the gels prior to forming the finished product formed from the polyolefin having the gels. One approach in particular is that of filtering the polyolefin melt. This is described in general in FILTRATION OF POLYMER MELTS (D. Gneuss, ed., VDI-Verlag GmbH, Düsseldorf 1981). In particular, Gerhard Schönbauer describes using screen mesh to filter polymer melts in this publication. However, there are no specific solutions to removing gels common in polyolefin melts, especially polyethylene melts and bimodal or multimodal polyethylene melts.
Others have disclosed the use of screen mesh to filter polyolefin melts. In one approach to solving the problem of gels in polyolefin products, U.S. Pat. No. 5,730,885 discloses as a preferred approach to use multiple screens, those coarser than 250 mesh, thus avoiding undesirable back pressure and ripping of the screens. In a second approach, on the other hand, U.S. Pat. No. 6,485,662 discloses that a single active mesh screen that is relatively large mesh, those finer than 200 mesh having a micron retention size range of from 2 to 70 microns, in combination with several small mesh passive screens is desirable. These solutions are both somewhat inadequate because the first requires multiple active screens, increasing cost and complexity as well as being limited to polypropylene in its usefulness, while the second approach has the problem of high pressure drop and propensity for screens to plug quickly.
The present inventors have surprisingly found that a single, relatively coarse mesh size active screen can adequately filter polyolefin melts to commercially desirable levels. The inventors have also found that one, two, three, or four active screens are useful in producing gel free bimodal polyethylenes.